Integrins are cell surface molecules that mediate important interactions between cells and between cells and the extracellular milieu. Integrins can adopt at least two different conformations on cell surfaces: a non-activated conformation that does not bind to the integrin ligand and an activated conformation that can bind the integrin ligand. Cellular signalling can cause integrins to alter their conformation from a non-activated conformation to an activated conformation. After activation, integrins bind in a specific manner to their cognate ligands on the surface of other cells, in the extracellular matrix, or that are assembled in the clotting or complement cascades.
Each integrin includes an α subunit and a β subunit. Over twenty different integrin heterodimers are known. Many integrins are selectively expressed on particular cells in the body. For example, a subset of integrins are selectively expressed on leukocytes.
Integrins on leukocytes are of central importance in leukocyte emigration and in inflammatory and immune responses. Two exemplary integrins on leukocytes are LFA-1 and Mac-1. LFA-1 (αLβ2) binds to a number of cognate ligands, including inflammation-associated cell surface molecules (ICAM), e.g., ICAM-1, ICAM-2, ICAM-3, ICAM-4, and ICAM-5. Mac-1 (αMβ2) binds ICAM-1, the complement component iC3b, and the clotting component fibrinogen.